Researchers at Keio University spent weeks showing laboratory mice footage of mouse porn, mouse fights, and mouse sniffing. They found out that mice can distinguish between social behaviors—and that they prefer seeing fighting to sex, unless drugs are involved.

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Biologists, psychologists, and sociologists have been working on how to classify behavior for decades. At Keio University, researchers wanted to know if mice could do what scientists do: sort behavior into categories. They described the results of their experiments in a recent paper in Animal Cognition.

At first, the researchers exposed mice to a continuous loop of video. They saw, in sequence, a clip of mice mating, a clip of mice fighting, and a clip of mice sniffing each other. After the mice in the experiment had gotten a good look, they were moved to a mouse multiplex, where their main enclosure was flanked by two different small compartments, each playing videos.

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One compartment would show one type of video, and the other would show another type of video. The researchers used the amount of time the mice spent in each compartment to judge their overall film preferences. Given a choice between sniffing and sex, the mice spent 41 percent of their time in the sex theater, and only 34 in the sniffing theater. Given the choice between sex and fighting, the mice spent 40 percent of their time in the fighting theater and only 35 percent of their time in the sex theater.

“Oh crap, I’m in the porn theater again.” Credit: Rama/Wikimedia

The scientists had something that they knew the mice would prefer to any kind of video: morphine. In a second experiment, they began to inject mice with morphine whenever they went into a specific theater. Half the mice got an injection when they went into the sex theater, and the other half got the injection when they went into the fighting theater.

The injection was necessary to see if the mice really could distinguish kinds of behavior. In this experiment, the mice were no longer shown one single clip of sex or fighting. They were shown a range of clips, and their chance to get a shot of morphine depended on their ability to recognize that a certain type of behavior—not just a specific clip—would get them their drugs.

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Soon the researchers found that mice who had been given morphine in the sex theater spent more time in the sex theater, even though the previous experiment showed a marked preference for fighting videos over sex. And the mice that had been given a shot in the fight theater still hung out in that theater, waiting for their fix. So mice are able to figure out that a broad range of clips can be classified under the category of “sex” or “violence,” even though they aren’t able to articulate the classification system. They just know what they like.

If the mice’s priorities—violence over sex and drugs over everything—seem depressing, take heart. The researchers don’t know why they prefer the fights. It could be that the mice want to emulate the fighters, or it could be that they’re studying the behavior closely because they sense danger. Moreover, the mice used in this experiment were all male, and all sexually inexperienced. Perhaps if they had tried sex before any of this started, they’d be more interested in seeing it onscreen.